[packagekit] Getting and setting the proxy server

[Richard Hughes]

A thorny issue: dealing with proxy servers.

First, the setters:

* By default, using the config file /etc/PackageKit/PackageKit.conf
* The session, using PolicyKit method SetProxy(ss) - see below...

Then the getters:

* For threaded backends you can do pk_backend_get_proxy_http() and
pk_backend_get_proxy_ftp() if your backend supports either of these
download methods.
* For spawned backends you need to use the environment http_proxy and
ftp_proxy variables - most backends don't need to do anything as this
will be inherited by wget for example.
* for DBUS backends there is a method send called SetProxy() which can
be used to setup proxy state. This is sent straight after Init() is
called.

The format of the proxy string is username:password at server:port -- it's
not ideal but will cover 99% of the user base.

Now, when the session wants to set the proxy (i.e. it's just been set in
gnome-network-preferences) do we want to store the data permanently or
just for that user session? I would think the latter, as the session
http username and password probably isn't the same for all users and
might be a disclosure problem. I guess http passwords are going out
cleartext anyway, so maybe it's not a huge problem. Ideas welcome.

I've added code for everything I've described here in master
(724c328d40acff42445531861e823516a219ac63), except the session stuff.

Ideas welcome.

Richard.